The overall objective is to more clearly define human health risks from prenatal exposure to methylmercury compounds (MeHg). To achieve this objective, we plan to measure dose-response relationships in human populations exposed to methylmercury and to conduct complementary investigations to characterize more precisely both prenatal and early postnatal body burdens of methylmercury including transport to the target organ, the brain. The aim of the human studies is to test the hypothesis, developed in our previous study of prenatal exposures in the Iraq population, the subtle psychological and behavioral changes in prenatally exposed children can be quantitatively related to the mothers exposure during pregnancy using dose- response models. MeHg concentration in head hair has been used as the best indicator of the dose. We plan to directly test the assumption that hair levels indicate levels of methylmercury in the target tissue, the brain, by use of human autopsy data. We plan to examine the mechanisms of transport of MeHg from blood to brain across the blood-brain barrier to better understand the factors that limit the accuracy of hair mercury as a biological monitor of dose to the target tissue. We have preliminary evidence that MeHg crosses the blood-brain barrier on the neutral amino acid carrier as a complex with the thiol- containing amino acid, cysteine.